codes is used when a new code format is introduced and the parameters in this code are
determined e.g. such that the same level of safety is obtained as in the old code or calibrated
to a target reliability level. In practical code optimization the following steps are generally
performed:
Definition of the scope of the code.
Definition of the code objective.
Definition of code format.
Identification of typical failure modes and of stochastic model.
Definition of a measure of closeness.
Determination of the optimal partial safety factors for the chosen code format.
Verification.
Ad 1. The class of structures and the type of relevant failure modes to be considered are
defined.
Ad 2. The code objective may be defined using target reliability indices or target probability
of failures. These can be selected from Tables Table 11.1 Table 11.2 depending on the use and
characteristics of the considered class of structure.
Ad 3. The code format includes: how many partial safety factors and load combination
factors to be used should load partial safety factors be material independent should material
partial safety factors be load type independent how to use the partial safety factors in the
design equations rules for load combinations In general for practical use the partial safety
factors should be as few and general as possible. On the other hand a large number of partial
safety factors is needed to obtain economically and safe structures for a wide range of
different types of structures.
Ad 4. Within the class of structures considered typical failure modes are identified. Limit state
equations and design equations are formulated and stochastic models for the parameters in the
limit state equations are selected. Also the frequency at which each type of safety check is
performed is determined.
The stochastic model for the uncertain parameters should be selected very carefully.
Guidelines for the selection can be found in JCSS (2001). Also in the Eurocodes (2001) and
ISO (1998) some guidelines can be found. In general the following main recommendations
can be made.
Strength or resistance parameters are often modelled by Lognormal distributions. This avoids
the possibility of negative realizations. In some cases it can be relevant also to consider a
Weibull distribution for a material parameter. This is especially the case if the strength is
governed by brittleness, size effects and material defects. The coefficient of variation varies
with the material type considered. Typical values are 5% for steel and reinforcement, 15% for
the concrete compression strength and 15-20% for the bending strength of structural timber.
The characteristic value is generally chosen as the 5% quantile.